Asynchronicity of fine sediment supply and its effects on transport and storage in a regulated river
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Purpose A disconnected ephemeral tributary was reconnected to the regulated River Ehen (NW England) as part of a river restoration initiative, providing a renewed delivery of sediment to a highly stable and armoured channel. This paper (1) assesses spatial and temporal dynamics of suspended and stored sediments in the Ehen, (2) characterises the composition of stored sediment, (3) develops fine sediment budgets for downstream river reaches, and (4) assesses the controls on the storage of fine sediment in the riverbed. Materials and methods A 3-km study section in the upper part of the River Ehen was divided into two reaches. Suspended sediments were monitored at the downstream limits of each reach over a 2-year period. In-channel storage was measured in three morphological units within the upper reach, on 13 occasions over the same period. Samples were used to assess changes in volumes of stored fine sediment, as well as the grain sizes and organic content of the material. A time-lapse camera facing the confluence of the tributary was used to conceptualise different flow scenarios. These scenarios reflect the degree of synchronicity between flows in the main-stem and those in the tributary. Fine sediment budgets were developed for each reach to assess the relative contribution of different sources of sediment. Results and discussion The reconnection significantly affected suspended sediment loads in the Ehen. Bed storage increased twofold, with changes most evident in the slow-flowing morphological unit. Changes in the composition of stored sediment were less marked than changes in the quantity of material. Changes in bed storage were controlled by the degree of synchronicity between flows in the Ehen and those in the newly reconnected tributary. Results show that three generalised flow scenarios occur, with total asynchronicity between flows in the tributary and the Ehen being responsible for the main episodes of fine sediment deposition. Overall, the estimated sediment budgets provide insights into the importance of non-perennial sources of sediment in supply-limited systems such as the Ehen. Although bed storage values are within the range of those published for UK rivers, the increase observed since the reconnection, together with the persistence of a static pavement, highlights the ecologically critical conditions of the regulated main-stem River Ehen. Conclusions Intermittent sources control fine sediment transport dynamics in the upper River Ehen. In this regulated river, ongoing deposition associated with increased low- and medium-sized flow events exerts more of a control on bed storage than large but rare floods. Management actions to limit delivery of material from lateral sources could help prevent further deterio- ration of habitat conditions for biota sensitive to fine sediment. Given the ongoing adjustment in the newly reconnected tributary, continued monitoring is needed to capture further morphosedimentary response in the main-stem.
Is part ofJournal of Soils and Sediments , 2018, vol. 18
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Geomorphological response to system‐scale river rehabilitation I: Sediment supply from a reconnected tributary Marteau, Baptiste; Gibbins, Chris N.; Vericat Querol, Damià; Batalla, Ramon J. (John Wiley & Sons Ltd, 2020-07-26)This paper is the first of a pair that report the findings of a river rehabilitation project centred on the reconnection of a formerly diverted headwater tributary (Ben Gill) to its main‐stem river (the River Ehen). The ...
Geomorphological response to system‐scale river rehabilitation II: Main‐stem channel adjustments following reconnection of an ephemeral tributary Marteau, Baptiste; Gibbins, Chris N.; Vericat Querol, Damià; Batalla, Ramon J. (John Wiley & Sons Ltd, 2020-07-26)This paper describes changes in bed morpho‐dynamics and topography in the River Ehen, a regulated river in NW England (i.e., temperate climate) following a rehabilitation project that reconnected a formerly diverted headwater ...
Marteau, Baptiste; Batalla, Ramon J.; Vericat Querol, Damià; Gibbins, Chris N. (John Wiley & Sons, 2017-07-21)Studies of ephemeral streams have focused mainly in arid and semi‐arid regions. Such streams also occur widely in temperate regions, but much less is known about their influence on fluvial processes in main‐stem rivers ...